Electric current is injected into a tunable filter to change the concentration of charge carriers, therewith changing the refractive index and, in turn, determining the centre frequency of the filter. Bragg filters and coupler filters are examples of such filters. Phase delay sections also function in accordance with the same principle.
One problem with elevating the concentration of charge carriers is that optical losses will also increase as a result of the absorption of free charge carriers. This increase in losses results in a reduction in transmission through the filter, which is often disadvantageous.
One solution is to use a semi-active material as tunable material, where increased absorption of charge carriers occurring when current is injected into the semi-active waveguide is caused to be essentially compensated for by amplification in the waveguide, this being achieved by choosing in the semi-active waveguide a material that will provide a sufficiently high amplification or gain.
One problem with semi-active material is that the amplification in this material takes place by stimulated emission, wherewith the optical power influences the number of charge carriers that remain in the material, i.e. free charge carriers. When the charge carriers interact too strongly with the optical field, too many charge carriers recombine and therewith cause an excessive reduction in charge carrier density, so as to make tuning of the filter impossible to achieve.
It may be difficult to decide an optimal interaction between the optical field and the charge carriers. This optimum depends on many variables, for instance the magnitude of the optical power prevailing in the laser at that moment in time.